PROJECT SUMMARY/ABSTRACT The androgen receptor (AR) signaling pathway is the key driver of human prostate cancer. Prostate cancers are almost always sensitive to blockade of the androgen receptor, and tumors with acquired resistance to these therapies are often sensitive to second-generation inhibitors of the pathway. However, the vast majority of men with progressive prostate cancer will ultimately succumb to the disease. Activation of the PI3K signaling pathway is the second most prevalent oncogenic event in prostate cancer, occurring in the majority of advanced castration-resistant tumors. The PTEN tumor suppressor gene is lost or inactivated in 40% of these tumors, and genetic events that activate PI3K are also found in a significant number of patients. While PTEN loss is the most commonly altered regulator of PI3K signaling in prostate cancer, recent genomic profiling studies of metastatic castration-resistant prostate cancer (CRPC) have reported activating alterations in the subunits of the PI3K complex (PIK3CA, PIK3CB, PIK3R1) in approximately 10% to 20% of cases. Our previous work revealed that activation of AR-signaling or PI3K signaling each cause feedback inhibition of the other in a reciprocal fashion. As predicted from this finding, inhibition of androgen receptor signaling activates PI3K signaling in these tumors and correspondingly, inhibitors of PI3K signaling activate AR signaling. It is likely that relief of feedback inhibition of the other pathway reduces the benefit of therapy with single agent AR or PI3K inhibitors. In support of this hypothesis, we showed that combined inhibition of both pathways has profound therapeutic effects in preclinical models of advanced prostate cancer. The major goal of this proposal is to optimize such therapies, tailoring the therapeutic strategy to specific tumor genotypes. We will do this by determining the biologic consequences of different mechanisms of PI3K activation in prostate cancer, determining the best means of pharmacologically inhibiting PI3K and AR signaling in prostate tumors with PTEN loss and/or PI3K activation, and optimizing the dose and schedule of these combinations in novel organoid and genetically engineered mouse models of prostate cancer. The aims of this proposal are to explore the biologic consequences of PI3K pathway activation in AR-driven prostate cancer and to determine the optimal combination regimens with which to treat these tumors.